1. Field of the Invention
The present invention relates to a mobile station unit which is movable from one of zones (communication areas) that are controlled by base station connecting units to another zone and capable of communication with another station through the connecting unit for a zone where it is located.
2. Description of the Related Art
A mobile radio system is a communication system in which individuals and moving vehicles carry mobile stations which are one type of radio equipment to thereby permit communication with a telephone network and between mobile stations. Being very convenient, the system has had a tendency to spread rapidly.
FIG. 6 is a schematic illustration of a mobile radio system. The system is constructed from a base station control unit A, base station connecting units BS-1, BS-2, . . . , BS-n, and mobile station units (e.g., cellular radios) PS-1, PS-2, . . . , PS-n.
The base station control unit A makes connection (communication) between a telephone network and each of the mobile station units through a respective connecting unit and connection between mobile station units present in zones that different connecting units cover. The connecting units BS-1 to BS-n control first to n-th zones, respectively, which are their respective service areas and have communication control of mobile station units within their respective zones. Even if each of the mobile station units PS-1 to PS-m, which are radio telephones, moves to any of the first to n-th zones, it is permitted to communicate with the connecting unit covering a zone where it is located. Transmission of information through control and communication channels between the connecting units BS-1 to BS-n and mobile station units PS-1 to PS-m is made on a time division multiplex (TDM) basis. Connection between one of the connecting units BS-1 to BS-n and mobile station units within the zone covered by it is made by time division multiple access (TDMA).
In such a system as described above, when a mobile station unit moves from a zone to another zone while in communication, it is required to switch between communication channels because the mobile station unit is switched from a connecting unit to another. Conventional mobile stations make the switching between communication channels as follows.
Before proceeding with a description of the switching between communication channels, it will serve us to explain physical slots of the connecting units BS-1 to BS-n.
FIG. 7 is a conceptual diagram illustrating one example of physical slots of the connecting units. That is, the physical slots of the connecting units comprise a sequence of superframes each of which consists of a plurality of frames as shown. More specifically, the superframe comprises ten frames. Moreover, each frame comprises eight slots. Of the eight slots, the first-half four slots indicated by Tx are allocated for transmission and the latter-half four slots indicated by Rx are allocated for reception. It should be noted that the above "transmission" and "reception" are intended to represent transmission from and reception by the connecting units. Thus, the relationship of the transmission and reception is reversed in the case of transmission from and reception by the mobile stations.
The leading slot of each frame is allocated for control channels. In each frame, the control channels are allocated to a broadcast channel (BCCH), paging channels PCH (in this example, there are four general calling channels, P1CH, P2CH, P3CH and P4CH), and a specific cell control channel (SCCH). That is, in FIG. 7, the frame control channel indicated by BC is used as BCCH, the frame control channels indicated by P1 to P4 are used as P1CH to P4CH, and the frame control channel indicated by SC is used as SCCH. The remaining slots in each frame which are indicated by T2, T3, T4, R2, R3, and R4 are communication channels.
Subsequently, the channel switching operation will be described.
Usually, the line quality is monitored during communication. However, once the deterioration of the line quality is detected, a communication channel in use is disconnected and the BCCH is retrieved. If the BCCH can be detected at this time, the self-PCH is calculated from a part of the system configuration parameters informed by the BCCH and from a predetermined self-ID, so that the corresponding PCH is retrieved. Next, upon receipt of a frame containing the corresponding PCH, the mobile station unit sends a link establish request to the connecting unit by the use of the control channel transmission slot (the fifth slot) of the same frame or the transmission slot (the fifth slot) of a subsequent frame containing SCCH. After the link has been established, a communication channel is set and communication is restored through the set communication channel.
There are two methods of setting frequencies of the control channels: fixed and variable (say, about 50 channels). In a system in which the control channel frequencies are fixed, when the deterioration of the line quality is detected, the receiving frequency is switched to a control channel frequency and then each channel is retrieved as described above. In a system in which the control channel frequencies are variable, the receiving frequency is sequentially changed at intervals of some tens of miliseconds to some hundreds of miliseconds (in the case of a frame period of 5 miliseconds), during which time the control channel is identified, and the BCCH is retrieved on the basis of a channel identifier. After the detection of the control channel, the same as described above is performed.
As described above, in the conventional mobile station unit, when the deterioration of the quality is detected, a phenomenon will occur in which communication is interrupted for a relatively long period of time that elapses from the initiation o receipt of the control channel to the time when a communication channel is set to restore communication after the detection of the BCCH, PCH and SCCH. The communication interruption period is particularly long in the case there the control channel frequencies are variable.
As described above, at the time of switching between channels when moving from a zone to another zone, the conventional mobile station unit is arranged to disconnect a communication channel and then receive control channels a point of time when the deterioration of the line quality is detected, thus making the communication interruption time long. This will make users unpleasant.